1. Field of the Invention
The present invention relates to a hermetic encapsulation package and a method of fabrication thereof. More particularly, it relates to a hermetic encapsulation package and a method of fabrication thereof, which are especially suitable for hermetically sealing organic EL (electroluminescent) elements used for plain light sources or display devices in which an electric field is applied to a light emitting layer of organic compounds to emit light therefrom.
2. Description of the Related Art
Electroluminescent elements (hereinafter referred to as EL elements) include inorganic EL elements and organic EL elements. Both EL elements are self luminescent and thus provide high visibility. In addition, since they are perfect solid-state elements, they provide improved resistance to a shock and can be handled easily. For these reasons, the EL element is being developed as pixels for use in graphic displays or television image display devices, or plain light sources. In particular, unlike the inorganic EL element, the organic EL element is not constrained to the requirements of alternating-current drive and high voltages. In addition, the variety of organic compounds employed by the organic EL element conceivably makes it relatively easy to provide multiple colors. Thus, application of the organic EL element for full-color displays or the like is expected and a structure of the organic EL element is being developed to provide high luminance at low voltages. The inorganic EL element is excited with an electric field to emit light. On the other hand, the organic EL element emits light by so-called carrier injection, in which positive holes are injected from the anode and electrons from the cathode for the operation of the organic EL element. The positive and negative carriers injected from both electrodes move to the opposite electrodes and are re-combined to form excitons therein. Relaxation of the excitons produces the emission of light which is in turn the light emission from the organic EL element.
The organic EL element was previously studied intensively using anthracene single crystal of high purity, however, such an organic EL element provided low luminance and low luminous efficiency with less stability irrespective of the necessity for the application of high voltages. However, in 1987, Tang et al from Eastman Kodak Company announced that they employed a two-layer stacked structure of organic thin film to provide a stable high-luminosity light emission at low voltages. Since then, the research and development of the organic EL element has been suddenly energized. In this structure, an organic layer sandwiched by a pair of electrodes was fabricated of two stacked layers of a light-emitting layer and a hole transport layer. The organic layer provides an outstanding property of 1,000 cd/m2 upon application of 10V (Tang et. al., Appl. Phys. Lett., 51 (12), 913 (1987)). These days, in some cases, provided are not only the light-emitting layer and the hole transport layer but also an electron transport layer between the cathode and the light-emitting layer. Alternatively, a hole injection layer may be interposed between the hole transport layer and the anode. In addition, various studies on the materials used for each layer have yielded many results concerning an increase in luminous efficiency and longevity, leading to an increasing expectation of applying the organic EL element for flat panel display devices with the organic EL elements being arranged in a matrix of X-Y plane. Thus, this has led to the development of a monochromatic passive matrix panel with 256xc3x9764 dots, a xc2xc VGA 5-inch diagonal color panel, and a VGA 10-inch diagonal color panel. (For example, reference can be made to Display and Imaging, Vol. 5, pp273-277 (1997) by Hitoshi Nakata et al; Fundamental and Practical Techniques of Organic EL Element by Hitoshi Nakata, the Japan Society of Applied Physics, Organic Molecular Bioelectronics Division, Sixth Lecture Notes, pp147-154 (1997); Flat Panel Display 1998, p234, Nikkei Business Publications, Inc., etc.)
The organic EL element has a structure in which an organic thin film of thickness equals to or less than 1 xcexcm sandwiched by two electrodes. Exposure of the element after fabricated to the surrounding atmosphere would cause the organic layer and the material of the electrodes to react somehow with gas (such as moisture and oxygen) in the atmosphere. This may cause non-light-emitting regions to appear on the element, referred to as dark spots, which degrade the light emission properties. Therefore, it is practically necessary to provide the element with an encapsulating mechanism to seal out the atmosphere immediately after the fabrication of the element. The sealing methods include a direct technique of providing a protective layer on the outer surface of the element or molding the element with resin or the like. An alternative method is an indirect technique of providing the element with a hollow structure using a cap or the like to seal out the atmosphere. In some cases, the element is directly protected and further provided with another sealing means.
Specific examples of the direct technique have been reported. In a method (Japanese Patent Laid-Open Publication No. Hei 3-37991), the element is molded with oxygen absorbent resin. In a method (Japanese Patent Laid-Open Publication No. Hei 7-169567), a gas barrier layer and a gas absorbent layer are employed at the same time. In a method (Japanese Patent Laid-Open Publication No. Hei 8-236271), the sealed space is filled with an elastic resin. In a method (Japanese Patent Laid-Open Publication No. Hei 8-124677), a stress relaxation layer is interposed between the rear surface and an adhesion layer of the organic EL element upon face-to-face bonding of a sealing material such as a glass plate to the rear surface of the organic EL element with the adhesive. In a method (Japanese Patent Publication No.2813495), an insulating polymer layer is provided on the outer surface of the element and further an insulating glass or the like is provided thereupon. In a method (Japanese Patent Publication No.2813499), an insulating inorganic compound material is provided on the outer surface of the element, a hygroscopic layer is deposited thereupon, and further an insulating glass layer or an insulating polymer film is provided thereupon. In a method (Japanese Patent Laid-Open Publication No. Hei 4-363890), the element is held in a fluorine-based inert liquid. In a method (Japanese Patent Laid-Open Publication No. Hei 5-182759), a non-permeable substrate is fixed to the element via a photo curing resin having moisture resistance. Finally, in a method (Japanese Patent Laid-Open Publication No. Hei 9-274990), a protective layer is provided on the outer surface of the element, the element is then molded with a sealing layer having a certain physical property, and a cap is further provided to seal out the atmosphere.
On the other hand, specific examples for the indirect technique previously reported include a method (Japanese Patent Laid-Open Publication No. Hei 9-148066) for fixing a desiccant within a hermetically sealed space in a spaced apart relationship from the element. Further included in the examples is a method (Japanese Patent Laid-Open Publication No. Hei 3-261091) for holding the element in a hermetically sealed space filled with a desiccant of phosphorus pentoxide (P2O5).
In the direct technique, either the protective layer is directly provided on the element or the element itself is molded directly with resin or the like. This causes great concerns to rise that the organic EL element formed of an organic film having a thickness of less than 1 xcexcm may be susceptible to damage. The damage may be caused not only by force or heat applied directly to the element during the process but also by stress or the like generated on the interface between the element and a protective agent. It is therefore preferable to employ a method in which the element is not directly touched. Furthermore, the direct technique presents a problem of requiring an additional step in the process and causing an increase in cost.
On the other hand, the indirect technique provides the element with a hollow structure using a cap or the like to seal out the atmosphere, thus having no adverse effect on the organic EL element and providing a relatively simple process. To seal the element with a cap or the like, a sealing cap is bonded to the support substrate of the organic EL element with an adhesive or the like. In this method, it is necessary to tackle the problem of how to enhance the adhesion level of the cap to the substrate, provided that the cap itself acts as an effective barrier to gases. The development of adhesive holds the key to solving the problem. Accordingly, research and development have been conducted aiming to provide the adhesive with higher adhesion, higher gas barrier property, and a lower amount of out-gas or the like. (For example, reference may be made to Critical Problems Remaining in Organic LED Device and Practical strategies, by Kenichi Horie, the Japanese Research Association for Organic Electronics Materials, published by Bunshin, pp98-103.) Incidentally, since the organic EL element is vulnerable to heat, it should be noted so as not to allow the temperature of the element to rise when a thermosetting adhesive is used for the element.
Such an adhesive has been currently in an advanced stage of development to provide a higher sealing effect, however, not sufficient sealing effect. However, it is possible to provide a relatively good durability for the element by providing the element with a sufficiently wide adhesion width for a cap to be fixed thereto. Nevertheless, providing an excessively wide adhesion width to the device would cause the light emission region to be narrowed for the total area of the device. This would become a significant drawback, for example, in the application for notebook-type personal computers. It is therefore an important theme to enhance the sealing effect without a considerable increase in adhesion width.
Furthermore, in the method (Japanese Patent Laid-Open Publication No. Hei 9-148066) for accommodating the desiccant within the hollow hermetically sealed space of the element, it is necessary to fix the desiccant in the space to prevent the desiccant from touching the element. This leads to an increase in the number of steps in the process and in cost.
An object of the present invention is to provide a hermetic encapsulation package and method of fabrication thereof at low cost, in which the sealing effect of the hermetic encapsulation package is enhanced by sealing the organic EL element with a cap to thereby prevent dark spots and particularly durability is improved to thermal shock.
A hermetic encapsulation package according to the present invention comprises a support substrate, at least one element mounted on said support substrate, and a sealing cap bonded to said support substrate accommodating said element to form a sealed space defined by said support substrate and said sealing cap. Said sealing cap deforms to vary a volume of said sealed space according to an increase or a decrease in pressure inside said sealed space.
According to the present invention, the volume of the sealed space can be varied to allow a change in pressure inside the sealed space caused by a change in temperature to be relaxed, thereby reducing the amount of gases passing through between the surrounding atmosphere and the sealed space. Thus, the hermetic encapsulation package according to the present invention can be provided at low cost in a reduced number of steps of the fabrication process thereof and sufficiently prevent the degradation of the element accommodated therein.
An organic EL device, which contains an organic EL element and to which the hermetic encapsulation package can be preferably applied, comprises a pair of electrodes and at least one organic EL element which is mounted on the support substrate and has an organic compound layer, including at least a light-emitting layer composed of an organic compound, sandwiched between the pair of electrodes. The organic EL device is characterized in that said support substrate and the sealing cap are bonded to each other, and said sealing cap deforms according to an increase or a decrease in pressure inside the sealed space defined by said support substrate and the sealing cap, thereby varying the volume of the sealed space.
A portion of the sealing cap, opposite to the support substrate, preferably deflects to vary a volume of the sealed space.
In addition, the sealing cap can increase and decrease the volume of the sealed space so as to relax a change in pressure caused by a change in temperature of a gas inside the sealed space in a manner such that the sealing cap is deflected outwardly relative to the sealed space with increasing temperatures to increase a volume of the sealed space and is deflected inwardly relative to the sealed space with decreasing temperatures to decrease the volume of the sealed space. The sealed space preferably provides a decrease in volume of 5% or more at xe2x88x9235xc2x0 C. and an increase in volume of 5% or more at 85xc2x0 C., relative to a volume at 25xc2x0 C.
Furthermore, at least part of the sealing cap is preferably formed of an elastic material of a Young""s modulus of 70 GPa or less.
Still furthermore, a gas barrier layer is preferably provided at least on one of the outer and inner sides of the sealing cap. The Young""s modulus of the gas barrier layer is preferably 70 GPa or less, which is the same as that of the sealing cap.
In addition, the support substrate and the sealing cap are preferably bonded to each other with an ultraviolet curing resin. The ultraviolet curing resin is preferably an epoxy-based resin.
A method for fabricating such an organic EL device comprises the steps of fabricating an organic EL panel having an organic EL element; and sealing for accommodating pixel-formed portion of the organic EL panel. The step of fabricating the organic EL panel comprises the step of depositing on a support substrate by sputtering or vapor deposition a predetermined pattern of an anode layer, an organic compound layer including at least a light-emitting layer composed of an organic compound, and a cathode layer in that order, or a predetermined pattern of a cathode layer, an organic compound layer including at least a light-emitting layer composed of an organic compound, and an anode layer in that order. On the other hand, the step of sealing includes the step of bonding a sealing cap, formed of an elastic material having a Young""s modulus of 70 GPa or less, to the support substrate in a dried atmosphere so as to accommodate a pixel-formed portion of an organic panel.